This proposal seeks to understand the molecular mechanisms which underlie the oncogenicity of Moloney murine sarcoma virus (M-MSV). These studies will examine the transcriptional control of the transforming gene of M-MSV, the mos gene, with respect to its parental transcriptional unit which is the env gene of Moloney murine leukemia virus (M-MLV). The mos gene, transduced from the Balb/c mouse genome by recombination, is actually expressed as an env-onc fusion protein, i.e. the first five codons of the reading frame are donated by the M-MLV env gen. The mos gene is transcribed poorly, in contrast to the env gene which is expressed by means of an abundant spliced subgenomic mRNA. Experiments will be carried out to identify and to rectify the transcriptional lesions in M-MSV by recombining the mos coding region with transcriptional control signals from nondefective M-MLV. This should result in overexpression of the mos gene product in eucaryotic cells and will facilitate an understanding of its transcriptional regulation. The second part of this proposal deals with the structure and function of the oncogene product, p37mos, encoded by M-MSV. The biochemical function of p37mos is unknown. Recombinant DNA techniques will be employed to produce biochemical amounts of p37mos in both bacteria and eucaryotic cells. These proteins will be purified and examined for their biological transforming activity. High titer cross-reacting anti-mos antisera will be prepared. The mos gene product also will be assayed for possible enzymatic functions in an effort to understand its function. The last part of this proposal aims to construct vectors for the expression of inserted eucaryotic genes in transmissible retroviruses, based on knowledge of the env gene of M-MLV. These vectors will function by insertion into the env gene transcriptional unit, such that the inserted gene will be expressed by means of an abundant spliced subgenomic mRNA as is the env gene. Two vectors will be constructed. The first will allow expression of the inserted gene as a fusion protein with the env gene product. The second vector will permit expression of the inserted gene as a nonfused gene product, initiating translation with its own ATG. These vectors will be evaluated by insertion of genes coding for the rat growth hormone, the polyoma middle-T antigen, as well as the mos gene of M-MSV.